1. Field of the Invention
This invention relates to light control systems for video cameras and more particularly to an automatic light control system for controlling the intensity of a light source as a function of the light reflected from a reference background onto a photosensor in a video camera.
2. Description of the Prior Art
There are two basic types of prior art, optical imager devices and systems for controlling the light reflected thereon. Such optical imagers include exposure meters, photographic cameras, video cameras, document scanners and other image pickup devices.
The first prior art type of optical imager changes the exposure time as an inverse function of the amount of light received by a photosensor in or associated with the optical imager. For example, the exposure time would be made longer for low-light reception or made shorter for high-light reception.
There are several disadvantages to the use of this first prior art type of optical imager. In industrial applications, such as in material processing where objects may be moving past an optical imager, the exposure time could not be changed without changing the speed at which the objects are moving past the optical imager. It is not feasible to do this because jams in a moving production line may result from such variable speeds. Also, in industrial applications the throughput, or production rate, would decrease because some people working along a moving production line could not keep up with a variable-speed production line.
Typical prior art devices and systems for controlling the exposure time of optical imagers are described in U.S. Pat. Nos. 3,717,077; 3,741,088; 3,944,816; 4,174,528 and 4,176,955.
The second prior art type adjusts the iris of the optical imager to increase or decrease the light forming on a photosensor in the optical imager in accordance with the operating requirements of the optical imager.
There are several disadvantages to the use of this second prior art type of optical imager. A change in the iris changes the depth of focus of the optical imager. It also changes the resolution of the optical imager.
Typical prior art devices and systems for changing the iris of the optical imager are described in U.S. Pat. Nos. 4,050,085 and 4,300,167.
In effect, each of the above-described types of prior art optical imagers compensates for the light intensity on an object being imaged by changing the incident light on the photosensor of the optical imager (by changing either the exposure time or the iris setting).
Thus, neither prior art type of optical imager teaches or even suggests a system and method for utilizing the output of the photosensor to automatically control the light intensity of a light source as a function of the incident light on the photosensor.